In spark ignited internal combustion engines, the combustion process normally exhibits cycle-to-cycle variability. This variability is known to result in such undesirable effects as engine roughness at idle and reduced engine efficiency at higher loads. Efficiency is reduced when peak combustion chamber pressure occurs at varying rotational locations on the crank circle.
Ignition delay variability is a major cause of cycle-to-cycle variations in combustion processes. Ignition delay is the time period between spark discharge and a measurable increase in combustion chamber pressure attributable to combustion This time period varies because of chaotic processes within the combustion chamber within the vicinity of the spark plug. These chaotic variations result from small scale mixture turbulence as well as small scale variations in mixture composition. As a result, from one combustion cycle to the next, the speed at which the combustion proceeds will appreciably vary, because variations in the turbulence and mixture composition near the spark plug gap will alter the speed with which the spark ignited flame kernel grows to a size which can influence the combustion chamber pressure.
One way to reduce variability in ignition delay is to increase the size of the spark. A larger spark will encompass a larger portion of the turbulent mixture and will tend to counteract some of the cycle-to-cycle mixture variability The overall time of ignition delay will also be reduced with a larger spark. Since a conventional spark is commonly 0.030 to 0.040 inches long (e.g., the size of the spark plug gap), the initial flame kernel ignited by this spark is quite small The surface area of this generally spherical flame kernel will grow as the square of the diameter of the sphere. Thus the surface area of the kernel will start out small but will begin to grow rapidly in an exponential fashion as its diameter increases. It follows that if the initial flame kernel is significantly larger, then the time it takes for the flame kernel to measurably affect combustion pressure will be reduced, and the total ignition delay time will be shortened In sum, variability in ignition delay can be reduced by a larger spark because small scale variations in fuel mixture composition will have less of an effect on a larger initial flame kernel, and overall ignition delay time will be reduced. A larger initial spark will result in a smoother running engine and will increase engine efficiency because peak combustion chamber pressure will occur at more consistent locations on the crank circle.
Various ways of increasing the size of the spark are known. Simply increasing the size of the spark plug gap is one method. However, the ignition system must be capable of providing sufficient voltage to fire the larger gap. Thus, if the spark gap is simply increased in a conventional ignition system, the increased voltage requirement may cause the engine to miss, especially at high rpms.
Another method of increasing the size of the spark is taught in U.S. pat. No 4,677,960. That invention teaches a magnetic field which moves the spark outward into the air/fuel mixture. This configuration utilizes the circuit comprising two parallel electrodes and the spark itself as a single turn solenoid or coil which produces a magnetic field. The spark will move to a lower energy condition, enlarging the area within the coil, to slightly reduce the flux density within the single turn loop. As a result, the length of the spark is increased from the linear distance between the two electrodes, to an arc shaped spark connecting the two electrodes. The effect on the flame kernel size, however will be minimal. Based on the shape and strength of the magnetic field produced in this manner, it can be expected that the length of this spark will probably increase by less than a factor of two Thus, simply bending the spark by means of a magnetic field will not have a major effect on the size of the spark and ultimately the cycle-to-cycle variability of ignition delay.
The present invention provides a spark plug which significantly increases the mixture volume traversed by the spark and thereby reduces cycle-to-cycle variability in ignition delay. It does this by incorporating a multiple turn coil or solenoid into the spark plug near the area of the spark gap. This solenoid creates a magnetic field which causes the spark to bend outward and also to rotate about the center electrode As the rotating spark sweeps around in a circular path, the resulting spark will traverse a volume of the mixture which is perhaps an order of magnitude greater than the spark in a conventional spark plug.
The actual surface area of the resulting spark path will be a function of the strength of the magnetic field, the angular speed with which the spark rotates about the center electrode and the current and duration of the spark discharge. A number of embodiments of the present invention are herein disclosed which provide various means for maximizing parameters and which result in an increase in the effective size of the spark. This has the effect of reducing cycle-to-cycle variability in ignition delay. Further, the more consistent location of peak combustion pressure on the crank circuit results in more efficient engine operation. An additional benefit of the present invention is that the engine will be able to run on leaner mixtures because the greater mixture volume traversed by the spark has an increased probability of comprising a combustible mixture among the small scale mixture nonuniformities.
In one form of this invention, a spark plug has a center high voltage electrode and an annular ground electrode concentric with, and surrounding the high voltage electrode. Also, an axial multiple turn solenoid surrounds the high voltage electrode near the spark gap. This solenoid carries current from the annular ground electrode to a conventional steel spark plug shell which is in electrical connection to ground. The solenoid creates a magnetic field perpendicular to the plane of the spark gap. This magnetic field has a steep intensity gradient that causes the spark to be bent outward from the gap plane. This happens because the spark is itself a current carrying conductor and will tend to move to a lower energy condition which is in the direction of the lesser intensity of the magnetic field. In addition, the magnetic force acting upon the spark will cause the spark to rotate about the high voltage electrode similar to the rotation of the spoke of a wheel. In completing one revolution, the spark will trace a shape similar to that of half of a circular torus, or donut, which has been sliced in the middle in a horizontal plane. Assuming one complete revolution, the total surface area of the half torus spark will be approximately S=pi.sup.2 RD; where D is the distance between the two electrodes and R equals the radius of the high voltage electrode plus 1/2D.
In another exemplary spark plug according to the present invention, a further enhancement of the magnetic field strength is achieved by the addition of a second coil or solenoid attached on one end to the high voltage electrode. The other end of the second solenoid is attached to the ignition wire. Consequently, ignition current passes through the second solenoid before it reaches the high voltage electrode. The magnetic field created by the second coil adds to the field produced by the first solenoid. As a result, the bending and the rotation of the spark is enhanced. The second coil connected to the high voltage electrode may be employed with or without the first coil connected to the ground electrode.
In yet another exemplary embodiment, the gap plane formed by the exposed surfaces of the high voltage and the ground electrodes is angled rather than perpendicular to the axis of the high voltage electrode. In this configuration, the gap distance is shorter on one side of the ground electrode than on the opposite side, due to the incline of the gap plane in the conical insulator section. As a result, the spark will initiate at the side with the shortest gap distance. This permits a lower sparking voltage to be utilized because the gap is smaller. Because of the well-known nonlinear impedance characteristic of a spark gap, once the spark has been initiated across the narrower portion, a lower voltage can sustain the spark across a wider portion of the gap as the spark is rotated by the magnetic field. The angled gap thus has the advantage of requiring a lower ignition voltage.
In yet another exemplary embodiment. a magnetic core may be inserted within the second coil. This magnetic core may be composed of a rod of magnetic material such as ferrite which is coated with an insulator. The purpose of the core is to further increase the strength of the magnetic field which is acting upon the spark
In another embodiment of this invention, a capacitor is integrated into the spark plug. This capacitor is connected electrically between the high voltage electrode and ground. This capacitor has the effect of increasing the intensity of the initial spark discharge to thereby produce a larger initial flame kernel. Ignition systems employing a capacitor for this purpose are sometimes known as "blast wave" systems and are described in S.A.E. papers Nos. 850076 and 880224. prior systems, however, employ a capacitor mounted externally to the spark plug. The present invention provides a capacitor which is monolithically built into the spark plug and the closer proximity of the capacitor to the spark increases the speed and initial intensity of its discharge.
Each of the above embodiments presents manufacturing difficulties that have not been overcome using conventional techniques for manufacturing spark plugs. To effectively utilize the various electrical components, such as coils and capacitors, required by these embodiments, involves more than merely attaching these components to a conventional spark plug. This is because these components must be in close proximity to the spark to be effective and therefore they are preferably integrated into the spark plug itself. To achieve this integration, techniques are taught for manufacturing these electrical components and the spark plug insulator as a single monolithic unit.
Generally, according to the present invention, the technique employed for integrating electrical components into a spark plug comprises a method for establishing electrically conductive monolithic paths through a solid by the use of a conductive ink. In particular, the method utilizes a cermet ink for creating conductive paths inside a solid insulating material such as a ceramic. The cermet ink is applied to the ceramic material at an early manufacturing stage when the ceramic is in a "green" state. This permits the ceramic insulator material to be co-fired with the cermet ink.
The cermet ink can be applied in patterns as desired depending on the desired electrical function. For example, to create a solenoid, a band of ink may be applied in a helical pattern around a cylindrical shaped portion of a green ceramic base. Additional layers of ceramic may then be applied over the coil to provide electrical insulation. To create a capacitor, a surface of cermet is first applied to a ceramic base Insulating ceramic then may be applied over the first surface and a second surface of cermet may be applied which is parallel to the first The resulting device, whether a capacitor or coil, may then be connected electrically to another component or wire by providing an inked surface at the terminal ends of the pattern which are suitable for such connections. In the context of this invention, the words "cermet ink" may mean any suitable fluid having an electrically conductive constituent and which is capable of forming an electrical conductor through solid insulator materials. In one example according to the present invention, the cermet ink comprises a ceramic and a metal suspended in a solvent.
These methods may also be successfully employed to manufacture spark plug electrodes. To manufacture the high voltage electrode, the cermet or other suitable ink is applied to a thin metal wire or spindle. This cermet coated spindle is then inserted into granulated ceramic material contained in a conventional rubber mold, such as the type used in the manufacturing of ceramic spark plug insulators and pressure is applied to the exterior of the mold. Because of the porous nature of the ceramic, the applied pressure causes the cermet ink coating the spindle to bond to the ceramic with a much stronger bond than the adhesive bond which initially held the ink to the spindle.
The spindle may then be carefully withdrawn from the ceramic material. As the spindle is withdrawn, the ink slides off the spindle and the hole left below the point of the spindle is filled in as the ink and ceramic material collapse due to the compressive forces maintained on the rubber mold. After the spindle is withdrawn, additional pressure is applied to further compact the ceramic body and the embedded cermet ink. This results in a strand of cermet ink running through the ceramic insulator which creates an electrically conductive path integrated into the insulator itself. The upper portion of the solid, high voltage electrode thus formed may then make direct contact with an ignition wire, preferably in a counterbore built into the upper insulator for receiving the wire. A small lower portion of the high voltage electrode may be coated with a platinum cermet which itself forms the high voltage electrode sparking surface.
In another embodiment of the present invention, a method for creating a ground electrode is disclosed which is similar to the above methods except that the conductive ink may be applied by dipping a small conical portion of the ceramic spark plug insulator tip into the ink. This method is particularly well suited to creating an annularly shaped ground electrode, such as the one which may be used in some of the above embodiments of this invention. In this method, the conical pointed lower tip of a ceramic insulator having an axial cermet high voltage electrode, in the "green" stage, is dipped into the cermet ink. After the insulator is fired, the tip of the cone is ground away by pressing it vertically on a horizontal grinding surface. This results in exposing an annularly shaped spark gap and ground electrode surface surrounding the high voltage electrode in the gap plane. Given a particular high voltage electrode, the gap distance will depend on the diameter of the insulator at the surface of the gap plane. By using an insulator with a small conical lower portion, the gap distance can be easily adjusted during manufacturing by varying the amount of material ground away from the insulator tip. The ground electrode may then be connected electrically to a conventional steel spark plug shell and hence to electrical ground, by means of a path of inked cermet connected to the ground electrode and to the spark plug shell bottom gasket
The above method of forming conductive paths through solid materials has the advantages of being relatively simple and cost effective to perform, and is readily adapted to mass production techniques. Moreover, by creating conductive paths which are integral with the solid, the overall reliability of the resulting device is improved because one integrated mass is employed rather than a set of discrete components. As a result, such a device can withstand large temperature extremes because there are fewer separate components having different coefficients of expansion. With respect to the manufacture of spark plugs, a further advantage of the above techniques is that they make it possible to integrate various electrical components, such a conductors, coils and capacitors, into the spark plug itself. This greatly facilitates the creation of a spark plug having a magnetic field in the area of the spark gap as is required by some of the embodiments of the present invention.